87Bumpstick

alright guys I have a 1987 Camaro 2.8 liter v6. I recently converted it to a manual, before the manual it was running and driving fine, for 2 months after the manual the car was running the best it ever has. now it wont start SOMETIMES the car will start right up and run fine, but when I turn it off then it wont start again until a few minutes and sometimes hours pass itll crank and spin spin spin but not start. I brought it to a mechanic he said my injectors weren't firing. he gave me three ways to fix it one was to change the ecm I did that and it didn't fix it, the next thing was the person that had the car before me had installed a security keypad, he told me to disconnect it so I did and still nothing, the third thing was a bad wire between the ecm and the injectors but I haven't looked into that yet. I have gotten a new battery, new coil new coil connectors new plugs new fuel filter have gotten the fuel pump checked it was fine new oil pressure switch EVERYTHING!! PLEASE HELP